Thermodynamic assessment of systems of actinide or rare earth with Cd

Abstract

A pyrometallurgical process is being developed for recycling nuclear reactor fuels. Thermodynamic information on multicomponent systems of actinides and rare earths (REs) with liquid Cd is very useful in the design of a process in which a liquid Cd electrode is used for the selective recovery of Pu and U, and a reductive extraction process using a molten salt/liquid Cd system for the recovery of minor actinides, such as Np, Pu, etc. A key issue in the design of these processes is a variation in solubility or activity of actinides or REs in multielement systems. In the present study, phase diagrams of U-Cd, Pu-Cd, Np-Cd, Y-Cd, La-Cd, Ce-Cd, Pr-Nd, Nd-Cd, and Gd-Cd were optimized by the CALPHAD method. For these systems, thermodynamic data, such as the activity of solutes in liquid Cd and the Gibbs energies of formation of the intermetallic compounds as well as the phase diagram data were available for the optimization. For optimization, the calculated primary results were entered into a database. Then, some ternary systems were preliminarily assessed through the use of the optimized data for the binary systems. Two extreme conditions were assumed: one condition was complete miscibility between the compounds that have the same mole ratio between solutes and Cd; the other condition was no solid solubility between the compounds. The results indicated the tendencies toward solubility and activity of actinides and REs in multielement systems.